(Three-Way Catalyst)
Exhaust gases of vehicles using gasoline as a fuel include harmful components such as total hydrocarbons (THC), carbon monoxide (CO), and nitrogen oxides (NOx). As such, it is necessary to purify each harmful component using a catalyst by oxidizing THC into water and carbon dioxide, by oxidizing CO into carbon dioxide, and by reducing NOx to nitrogen.
As catalysts for treating such exhaust gases (hereinafter referred to as “exhaust gas purification catalysts”), three-way catalysts (TWCs) enabling reduction-oxidation of CO, THC, and NOx are used.
As such a three-way catalyst, a catalyst adapted to load a noble metal on a refractory oxide porous body having a wide specific surface area, for instance, an alumina porous body having a wide specific surface area, and to load this material on either a substrate, for instance, a monolithic substrate made of a refractory ceramic or metal honeycomb structure, or refractory particles is known.
(OSC Material)
In this type of three-way catalyst, the noble metal preferably functions to oxidize hydrocarbon in the exhaust gas into carbon dioxide and water, to oxidize carbon monoxide into carbon dioxide, and to reduce nitrogen oxides to nitrogen, and maintains a ratio of air to fuel (air fuel ratio) in a constant way (to a theoretical air fuel ratio) in order to effectively produce catalysis for both of the reactions at the same time.
In internal combustion engines of, for instance, vehicles, the air fuel ratio is greatly changed depending on driving circumstances such as acceleration, deceleration, low-speed driving, and high-speed driving. As such, the air fuel ratio (A/F) that varies according to operational conditions of the engine is constantly controlled using an oxygen sensor (zirconia). However, since it is difficult for the catalyst to sufficiently exert purification catalyst performance in the case of merely controlling the air fuel ratio (A/F) in this way, a function of controlling the air fuel ratio (A/F) is also required of a catalyst layer itself. Thus, for the purpose of preventing a decrease in purification performance of the catalyst, which is caused due to a change in the air fuel ratio, using a chemical action of the catalyst itself, a catalyst in which a promotor is added a noble metal that is a catalyst active component is used.
As such a promotor, a promoter (called an “OSC material”) having an oxygen storage capacity (OSC) to release oxygen in a reduction atmosphere and to absorb oxygen in an oxidation atmosphere is known. For example, ceria (cerium oxide, CeO2) or ceria-zirconia composite oxide is known as the OSC material having the oxygen storage capacity.
Ceria (CeO2) has a characteristic that extends a range (window) of the air fuel ratio capable of efficiently purifying CO, THC, and NOx, and in which desorption and absorption of attached oxygen and lattice oxygen in the cerium oxide can be performed depending on a level of an oxygen partial pressure in the exhaust gas. In other words, when the exhaust gas has a reducing property, the cerium oxide desorbs the oxygen (CeO2→CeO2-x+(x/2)O2) to feed the oxygen into the exhaust gas, thereby causing an oxidation reaction. On the other hand, when the exhaust gas has an oxidizing property, the cerium oxide reversely takes the oxygen in oxygen deficiency (CeO2-x+(x/2)O2→CeO2), reducing an oxygen concentration in the exhaust gas to cause a reduction reaction. In this way, the cerium oxide fulfills a function as a buffer that decreases a change in the oxidizing and reducing properties of the exhaust gas, and has a function of maintaining the purification performance of the catalyst.
Further, the ceria-zirconia composite oxide that causes zirconia to be dissolved in this ceria is added to many catalysts as the OSC material, because the oxygen storage capacity (OSC) thereof is better.
With regard to the three-way catalyst using the OSC material such as ceria or ceria-zirconia composite oxide, the following inventions have hitherto been disclosed.
For example, in JP H06-219721 A, as a metal-metal oxide catalyst having a new catalyst characteristic, a catalyst that uniformly contains metal particles in metal oxide particles and contains any of Pt, Pd, Rh, and Au as a noble metal and CeO2 as a metal oxide is disclosed.
In JP 2011-140011 A, as a CO oxidation catalyst capable of showing a CO oxidation activity at a wide range of temperatures including a low temperature, a CO oxidation catalyst that carries Pd on CeO2 carrier particles and is formed by heat treatment in an oxidizing atmosphere at a temperature ranging from 850 to 950° C. is disclosed.
In JP H10-277394 A, as a catalyst that contains only palladium having a high conversion rate of hydrocarbon, carbon monoxide, and nitrogen oxide and excellent heat and aging resistance, a vehicle exhaust gas catalyst having a) fine active aluminium oxide stabilized, b) at least one fine oxygen storage component, c) and additionally high-dispersivity cerium oxide, zirconium oxide, and barium oxide, and d) a coat layer having catalysis of one layer made of palladium as a single catalysis noble metal on an inactive substrate is disclosed.
In JP 2005-224792 A), as a three-way catalyst in which Pd is carried on a support material based on a composite oxide containing Al, Ce, Zr, Y, and La, a catalyst in which a ratio BA of the total number B of moles of Al, Ce, Zr, Y, and La atoms to the number A of moles of Al atoms in the support material is 1/48 or more and 1/10 or less, a part of Pd is in a metal state, and a balance is in an oxide state is disclosed.
In JP 2010-521302 W, a three-way catalyst configured to apply strontium oxide or barium oxide to a surface of a catalyst layer made of aluminium oxide, cerium/zirconium mixed oxide catalytically activated by rhodium, and cerium/zirconium mixed oxide catalytically activated by palladium is disclosed.
(Exhaust Gas Purification Catalyst for Two-Wheeled Vehicle)
Incidentally, an exhaust gas purification catalyst for a two-wheeled vehicle has special problems different from those of an exhaust gas purification catalyst for a four-wheeled vehicle. For example, in comparison with the exhaust gas purification catalyst for the four-wheeled vehicle, the exhaust gas purification catalyst for the two-wheeled vehicle is required to have a small capacity and yet to exert a high purification capacity because of a limited space in which the catalyst is mounted.
Further, the two-wheeled vehicle makes heavy use of fuel because there is a tendency to emphasize output. Accordingly, an oxygen concentration in the exhaust gas is reduced, and thus the air fuel ratio (A/F) is frequently less than the theoretical air fuel ratio of 14.5. For this reason, even in the fuel-rich exhaust gas in which the air fuel ratio (A/F) is less than 14.5, it is required to efficiently purify the exhaust gas.
In this way, the exhaust gas purification catalyst for the two-wheeled vehicle has the special problems different from those of the exhaust gas purification catalyst for the four-wheeled vehicle. As such, the following proposals are made with respect to the conventional exhaust gas purification catalyst for the two-wheeled vehicle.
For example, in JP 2001-145836 A, as an exhaust gas purification catalyst that suppresses detachment of a catalyst layer caused by heat or vibrations to the minimum extent, suppresses thermal degradation of a catalytic material as well, and exerts good purification performance under severe conditions, there is proposed an exhaust gas purification catalyst that contains a metal carrier made of a cylindrical heat-resistant stainless punching metal, an undercoating layer of heat-resistant inorganic oxide to which an oxygen occlusion material placed on a surface of the carrier is added, and a catalyst layer placed on the undercoating layer, and that uses cerium oxide or a composite oxide of cerium and zirconium as the oxygen occlusion material.
In JP 2010-227739, as an exhaust gas purification catalytic material in which a catalytic activity is rarely reduced even in a two-wheeled vehicle exhaust gas atmosphere characterized by a drastic change in concentrations of oxygen, and HC and CO of unburned gas components in an exhaust gas and an excessive wind width of A/F, there is proposed an exhaust gas purification catalytic material having a carrier made of a cerium-zirconium based composite oxide containing 45 to 70 mass % of CeO2, 20 to 45 mass % of ZrO2, 2 to 20 mass % of Nd2O3, and 1 to 10 mass % of La2O3, and a catalyst component made of metal Pd or Pd oxide carried on the carrier.
In JP 2010-58069 A, there is proposed an exhaust gas purification catalyst for a two-wheeled vehicle, which is made up of a honeycomb-shaped carrier substrate having a plurality of cell passages, and a catalyst coating layer formed inside the cell passages of the carrier substrate, and in which the carrier substrate is divided into a front stage and a rear stage, and as a catalyst noble metal, palladium and rhodium are carried on the catalyst coating layer of the front stage, and rhodium is carried on the catalyst coating layer of the rear stage.
In JP 2011-20013 A, there is proposed an exhaust gas purification catalyst which is capable of efficiently purifying carbon monoxide and hydrocarbons even in a fuel-rich exhaust gas which a vehicle such as a two-wheeled vehicle emits and in which an air fuel ratio (A/F) is less than 14.5, and which contains cerium oxide, zirconium oxide, aluminium oxide, yttrium oxide, and/or magnesium oxide, and a noble metal, and the total concentration of yttrium and/or magnesium is 2.0 wt % to 5.0 wt % with respect to the total amount of the catalyst.
While vehicle driving, an oxygen excess condition (lean burn condition) on which an oxidation reaction is favorable and a fuel excess condition (rich burn condition) on which a reduction reaction is favorable alternate with each other with repetition in response to driving conditions. For this reason, it is necessary for the catalyst for the exhaust gas to exert predetermined catalyst performance or more under any of the oxygen excess condition (lean burn condition) and the fuel excess condition (rich burn condition). Especially, in the case of the two-wheeled vehicle, there is a tendency to raise the number of revolutions of an engine to drive the vehicle under a fuel-rich atmosphere. As such, it is required to exert the catalyst performance under the fuel-rich atmosphere.
On the other hand, in the existing three-way catalyst, platinum (Pt) and rhodium (Rh) among noble metals have been frequently used as the catalyst active components. However, since the prices of these noble metals are extremely high, development of a palladium catalyst using less expensive palladium (Pd) in quantity is required.
However, when palladium (Pd) was used as the catalyst active component, the purification performance as the exhaust gas purification three-way catalyst was shown to be good on a region from a theoretical air fuel ratio (stoichiometry) (A/F=14.7) to a lean air fuel ratio (A/F=14.7 or more) but, particularly, the purification performance of carbon monoxide (CO) was apt to be remarkably reduced under a fuel-rich atmosphere (A/F=less than 14.5), compared to the case of using rhodium and platinum as the catalyst active component.
Therefore, an object of the invention is to provide a new palladium catalyst capable of efficiently purifying carbon monoxide (CO) and hydrocarbons (THC) under a fuel-rich atmosphere even when palladium (Pd) is used as a catalyst active component.